Pistons having a reinforcing member in the top piston ring groove position are well known. Such members are used to reduce mutual wear between the piston ring groove and the co-operating piston ring. Frequently, where aluminium alloy pistons are concerned, the reinforcing member is made from a cast-iron such as Ni-resist (trade mark) in order to minimise the difference in coefficient of linear thermal expansion between the piston material and the material of the reinforcing member. In piston production, the reinforcing member is usually positioned within a female die mould member and molten aluminium alloy poured around the member. If the piston is being gravity die cast or pressure cast, the member is frequently pretreated by the well known Alfin (trade mark) process whereby it is preheated in a bath of molten aluminium to produce a wetted layer which includes aluminium-iron intermetallic compounds at the interface. A problem with this is that the bond formed between the piston material and the reinforcing member is very brittle and in-service failures of the bond in highly rated diesel engines, for example, are not uncommon.
An improvement over the Alfin technique is disclosed in GB-A-2,221,176 wherein the reinforcing member is first coated with a layer such as a stainless steel powder by a physical vapour deposition process such as plasma spraying, for example. The technique described involves coating at least those faces of the member which are contained within the body of the cast piston. The casting technique employed is pressure casting such as by squeeze-casting, for example. A problem with this method is that each individual reinforcing member needs to be sprayed on at least three faces, when viewed in cross-section, the faces being upper and lower, axially spaced-apart, radially directed faces and a radially inner, generally, axially directed face joining the two radially directed faces. This technique requires either manipulation of the reinforcing member or of the plasma spraying equipment and is time and materials consuming and, therefore, relatively expensive.
However, the method described does give an increase in bond strength of up to 100% over the earlier Alfin technique and a consequent increase in in-service durability of the bond.
We have now discovered that it may not be necessary in every application to coat every face of the reinforcing member which is contained with the piston body. We have found that it may only be necessary to coat the generally axially directed face to secure a significant improvement in in-service durability of the bond between the member and the piston body, over that of the earlier Alfin technique.
According to a first aspect of the present invention there is provided a piston having therein at least one piston ring groove reinforcing member, the at least one reinforcing member being annular in form and having, in cross-section, upper and lower, generally radially extending face portions and a radially inner, generally axially extending face portion between the two generally radially extending face portions, the generally axially extending face portion initially being coated with particulate material by a physical vapour deposition technique to provide a porous adhesion assisting layer, subsequently piston body alloy being cast around the reinforcing member and infiltrating the porous layer.
For the purpose of definition, the term generally axially extending face portion is defined relative to the piston axis and, the face portion may, for example, be planar, curved or may have two or more facets and may be taken to include the area adjacent the junction with the two generally radially directed face portions. The reinforcing member may, for example, be rounded in cross-section and may not have clearly defined boundaries to the face portions. In this case, the face portions may comprise arcs of the rounded cross section periphery with no clearly defined boundary therebetween.
The reinforcing member may be of rounded cross section; and may be of such a form as to reinforce two piston ring grooves with a single member.
According to a second aspect of the present invention there is provided a method of making a piston having at least one piston ring groove reinforcing member, the at least one reinforcing member having upper and lower, generally radially extending face portions and a generally axially extending face portion between the two generally radially extending face portions and radially inwardly of the piston outer diameter, the method comprising the steps of coating with particulate material the generally axially extending face portion by a physical vapour deposition technique to form a porous adhesion assisting layer, preheating the coated reinforcing member, placing the reinforcing member in a piston casting die, pouring molten piston body alloy around the reinforcing member, and allowing the molten alloy to solidify under an applied pressure causing the alloy to infiltrate the porous layer.
The thickness of the adhesion assisting layer may preferably lie in the range from 0.025 mm to 0.3 mm and more preferably from 0.05 mm to 0.15 mm, but is not considered to be particularly critical. At greater thicknesses the cost of deposition increases unnecessarily for no gain in bond strength and at lower thicknesses the full potential bond strength may not be developed.
The upper and lower, generally radially extending face portions may be bonded to the cast part of the piston body by being provided, after the provision of the coating of particulate material, with a wetted layer which includes an aluminium-iron intermetallic compound at the interface therewith. In such circumstances, the two generally radially extending face portions are, in effect, being treated by the Alfin technique. Thus, before the coated reinforcing member is placed in the piston casting die, the reinforcing member is dipped into a bath of molten aluminium to wet the upper and lower generally radially extending face portions. The coated reinforcing member may be preheated before being placed in the piston casting die by being dipped in the bath of molten aluminium for a time sufficient for the reinforcing member to be brought to the temperature of the bath.
We have found that the Alfin type bond is, in most instances adequate in compression and tension but inferior in shear. Therefore, the radially directed faces may be Alfin bonded but the axially directed face needs an improved bonding technique of the type described in GB-A-2,221,176. It has been found that crack initiation at the bond usually occurs at the axially directed face and may propagate to the radially directed faces.
The particular material may be oxidation; and/or corrosion; resistant, for example, stainless steel, e.g. 316L type. The corrosion resistance may be necessitated if preheating in a bath of molten aluminium is employed.
The adhesion assisting layer may be of a porous ferrous material, for example, the porosity stemming from the method of deposition.
Methods of physical vapour depositing the coating may include arc spraying, or plasma spraying, or flame spaying, for example.
Preferably, two or more reinforcing members, which are generally in the form of annular rings, are held together in an axial stack and in such an assembly are coated on their radially inner, generally axially extending face portions. The coating of the generally axially directed face may be effected by traversing a plasma spray gun, for example, mutually axially with respect to the rings whilst the stack and plasma gun are mutually rotated. Since a stack of reinforcing members may be coated simultaneously, the cost and inconvenience of the coating step may be greatly reduced, thus making the cost of piston manufacture more economical.
In order that the present invention may be more fully understood, examples will now be described by way of illustration only with reference to the accompanying drawings.